The present disclosure relates generally to turbomachinery, particularly to gas turbine engines, and more particularly, to an acoustic damping apparatus to control dynamic pressure pulses in a gas turbine engine combustor.
Acoustic pressure oscillations or pressure pulses may be generated in combustors of gas turbine engines as a consequence of normal operating conditions depending on fuel-air stoichiometry, total mass flow, and other operating conditions. Gas turbine combustors are increasingly operated using lean premixed combustion systems in which fuel and air are mixed homogeneously upstream of the flame reaction region to reduce oxides of nitrogen or nitrous oxides (NOx) emissions. The “lean” fuel-air ratio or the equivalence ratio at which these combustion systems operate maintains low flame temperatures to limit production of unwanted gaseous NOx emissions. However, operation of gas turbine combustors using lean premixed combustion systems is also associated with combustion instability that tends to create unacceptably high dynamic pressure oscillations in the combustor which can result in hardware damage and other operational problems. Pressure pulses resulting from combustion instability can have adverse effects on gas turbine engines, including mechanical and thermal fatigue to combustor hardware.
Aircraft engine derivative annular combustion systems that include relatively short and compact combustor designs are also vulnerable to the production of complex predominant acoustic pressure oscillation modes within the combustor. These complex acoustic pressure oscillation modes are characterized as having a circumferential mode coupled with standing axial oscillation modes between two reflecting surfaces. Each of the two reflecting surfaces is located at an end of the combustor corresponding to compressor outlet guide vanes (OGV) and a turbine nozzle inlet. The complex acoustic pressure oscillation modes create high dynamic pressure oscillations across the entire combustion system.
A number of existing approaches attempt to inhibit the development of unwanted pressure pulses during the operation of gas turbine engine have had limited success. Pressure pulses within a gas turbine engine combustor may be ameliorated by altering the operating conditions of the gas turbine engine, such as elevating combustion temperatures, which results in an undesirable elevation of NOx emissions. Other existing approaches make use of complex and potentially unreliable active control systems to dynamically control dynamic pressure pulses within a gas turbine engine combustor by producing cancellation pressure pulses in response to detected combustor pressure pulses detected by sensors installed within the combustor. Other existing approaches make use of passive pressure dampers such as holes perforating the liner of the combustor and/or detuning tubes positioned at various locations. However, passive pressure dampers are effective only specific fixed amplitudes and frequencies, rendering passive pressure dampers of limited use due to the varying amplitudes and frequencies of pressure pulses within a combustor. In addition, existing passive pressure damper designs project through openings formed through liner of the combustor, creating structurally vulnerable regions of high thermal stress.